Present data suggest that morphogenetic factors regulating connective tissue differentiation in mollusk embryos are lodged in the cortical cytoplasm of the polar lobes which form during meiosis and early cleavage of the fertilized eggs. The exact nature of these factors remains unknown. When the cytoplasmic polar lobe appearing at first cleavage is removed, the pathology of the resulting abortive "lobeless" embryo suggest that major deletions or alterations have occurred in the biosynthesis of many structural proteins and protein- polysaccharides. Fertilized eggs of the marine mud snail, Ilyanassa obsoleta, will be studied by electron microscopic and biochemical means in order to reveal the components of the vegetal hemisphere cytoplasm responsible for polar lobe formation and to elucidate the nature of the extracellular matrix defects which appear during the development of the lobeless embryos. Two specific questions will be asked. 1) Are microfilaments and microtubules involved in the formation and relaxation of polar lobe constrictions? 2) Are the structural proteins and protein-polysaccharides synthesized by lobeless embryos qualitatively and quantitatively the same as those made by normal embryos? Currently available techniques are adequate to test the degree to which the formation and resorption of polar lobes are dependent on the function of other organelles. Moreover, discovery of factors in the polar lobe cytoplasm which could induce or repress the synthesis of specific extracellular macromolecules would have important implications for the differentiation of vertebrate connective tissue.